This invention relates generally to tuners, and more particularly, to UHF television tuners of the detented type.
Detented UHF television tuners are known. Such detented tuners usually employ a UHF tuner having a continuously variable main tuning shaft and a detented channel selector shaft. A gear mechanism is generally used for mechanically coupling the selector shaft to the main tuning shaft. In addition, a fine tuning shaft is usually mechanically coupled to the main tuning shaft through a gear mechanism or the like, and serves to rotate the main tuning shaft over a predetermined range of rotation about each detent position established by the channel selector shaft. A clutch is generally interposed between the channel selector shaft and the main tuning shaft to permit the main tuning shaft to be rotated by the fine tuning shaft while the channel selector shaft is constrained by the detenting mechanism. Three such prior art tuners are described in U.S. Pat. Nos. 3,774,459; 3,842,683; and 3,916,820, incorporated herein by reference.
While these systems provide a UHF television tuner having a detented selector shaft and a fine tuning shaft for fine tuning the tuner at each of the detent positions of the selector shaft, such tuners suffer from a phenomenon known as inertial creep. The inertial creep phenomenon is caused by a rapid acceleration and deceleration of the selector shaft as it is rotated from one detent position to another, causing gradual slippage of the clutch coupling the main tuning shaft to the selector shaft, thereby resulting in a gradual detuning of the tuner after repeated channel changes.
One way to eliminate the inertial creep problem is to provide a fine tuning mechanism that operates independently of the main tuning shaft. This permits the main tuning shaft to be coupled to the channel selector shaft directly without the use of a clutch and eliminates the clutch slippage problem. A circuit for electrically achieving the independent fine tuning utilizing a voltage variable capacitor to adjust the tuning of the local oscillator of the UHF tuner is described in the above-referenced U.S. Pat. No. 3,842,683. Another system utilizing a mechanically operated piston capacitor to tune the local oscillator of the UHF tuner is described in U.S. Pat. No. 3,972,240 also issued to the same inventor as the inventor of the present invention and assigned to the present assignee.
While the above-described systems do provide a way to eliminate the inertial creep problem, the electrical fine tuning system described in U.S. Pat. No. 3,842,683 has a tuning range that is limited by the capacitance range of the voltage variable capacitor, and which is highly dependent on the physical layout of the circuit. The mechanical system described in U.S. Pat. No. 3,972,240 is somewhat cumbersome and therefore not suited to portable applications. Furthermore, the use of a piston capacitor increases the cost of the tuner, thus making it impractical for use in low cost television receivers. Also, the mechanism for adjusting the piston capacitor tends to have some degree of backlash, thus affecting the resettability of the tuner.